Circuit manufacturing and design techniques for reference plane voids with strip segment

ABSTRACT

Manufacturing circuits with reference plane voids over vias with a strip segment interconnect permits routing critical signal paths over vias, while increasing via insertion capacitance only slightly. The transmission line reference plane defines voids above (or below) signal-bearing plated-through holes (PTHs) that pass through a rigid substrate core, so that the signals are not degraded by an impedance mismatch that would otherwise be caused by shunt capacitance from the top (or bottom) of the signal-bearing PTHs to the transmission line reference plane. In order to provide increased routing density, signal paths are routed over the voids, but disruption of the signal paths by the voids is prevented by including a conductive strip through the voids that reduces the coupling to the signal-bearing PTHs and maintains the impedance of the signal path conductor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. patent application is a Division of U.S. patentapplication Ser. No. 12/823,316 filed on Jun. 25, 2010, now U.S. Pat.No. 8,325,490 which is a division of U.S. patent application Ser. No.12/015,543 filed on Jan. 17, 2008 now U.S. Pat. No. 7,821,796. Thepresent U.S. patent application is also related to U.S. patentapplication Ser. No. 11/751,786 entitled “MULTI-LAYER CIRCUIT SUBSTRATEAND METHOD HAVING IMPROVED TRANSMISSION LINE INTEGRITY AND INCREASEDROUTING DENSITY”, filed on May 22, 2007 by the same inventors andassigned to the same Assignee, and issued as U.S. Pat. No. 7,646,082 onJan. 12, 2010. The disclosures of the above-referenced U.S. patentapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to integrated circuit internalpackage interconnects, and more particularly, to a methodology andmulti-layer substrate that has improved signal integrity and impedancematching.

2. Description of the Related Art

High-density interconnect schemes for processor packages, as well asother very-large-scale integrated (VLSI) circuits typically use a largenumber of circuit layers to connect one or more dies to electricalterminals disposed on one or more surfaces of the package, as well as tointerconnect multiple dies in multi-die packages.

A typical stack-up for a present-day VLSI circuit substrate isfabricated in very thin layers on one or both sides of a rigid core thatprovides stiffness and stability to integrated circuit substrates, whichmay then be encapsulated after dies are attached. The core typicallyincludes pass-through vias that have a larger diameter than the viasused between the thin circuit layers and that pass between thininsulating layers. For example, in a substrate having a core 800 μmthick, the diameter of the through vias may be 500 μm in diameter, whilethe outer layer interconnects may have vias only 50 μm in diameter. Thereason for the larger diameter holes through the core is the relativethickness of the core, which makes reliable fabrication andresin/conductive filling of the vias more difficult than for viasbetween the thin insulating layers in the outer circuit layers that arelaminated on the core.

Since the interconnect routing density directly determines the requiredsize of the final package, routing resources are critical in anintegrated circuit package and space is at a premium. However, forcritical signal paths such as clock and high-speed logic signaldistribution, transmission lines must be maintained throughout thesignal path in order to prevent signal degradation. Therefore, areference voltage plane (e.g., ground) metal layer is provided on thesurface of the core, with voids around the via and interconnect areas atthe surface(s) of the core so that a transmission line is provided forthe next signal layer above/below the core surface metal layer(s). As aresult, signal path conductors must be routed around the large diametervias passing through the core which are not connected to the metallayer. Further, the signal path conductors must also be routed away fromdiscontinuities in the metal layers(s) caused by the voids through whichthe vias pass, since the lack of reference voltage plane metal willcause a change in impedance of the transmission line. Therefore, thenumber of signal routing channels is severely limited by the presence ofthe large-diameter vias that extend through the core that provide signalpaths, and the large-diameter vias that provide voltage planes otherthan the voltage plane connected to the core surface metal layer.

The above-incorporated U.S. patent application provides additionalrouting channels by providing continuous reference plane metal adjacentto conductive signal paths, and frees additional signal routing channelsover reference voltage vias by providing reference plane metal betweenthe ends of the vias and any signal paths routed above or below the viaends. Disruption of signals carried by signal-bearing vias is avoided byproviding voids in the reference plane metal above or below the ends ofthe signal-bearing vias. However, in such designs, routing is stillcritically limited by the inability to route signal paths over thesignal-bearing vias.

It is therefore desirable to provide a multi-layer integrated circuit,substrate and method that maintain signal integrity and impedancematching in an integrated circuit package while providing an increasedamount of signal routing channels, including channels routed oversignal-bearing vias.

SUMMARY OF THE INVENTION

The objective of improving signal integrity and impedance matching in amulti-layer integrated circuit substrate while permitting routing oversignal bearing vias is provided in an integrated circuit substrate, andmethods for making and designing the integrated circuit substrate.

The substrate includes a core having large diameter vias and at leastone signal layer having signal conductors having a width substantiallysmaller than the diameter of the large diameter vias. The signalconductors are connected to large diameter vias by a small diameterportion passing through a first insulating layer disposed between thecore and a transmission line reference plane metal layer, and a secondinsulating layer disposed between the transmission line reference planemetal layer and the signal layer.

The transmission line reference plane metal layer defines voids havingan area larger than the area of signal-bearing large diameter vias, sothat the presence of the transmission line reference plane metal layerdoes not cause substantial insertion capacitance with respect tocritical signals. For signal-bearing vias over which critical signalpaths are routed, a conductive stripe extends across the voids toisolate the critical signal conductive paths from the ends of thesignal-bearing vias. The width of the stripes may be equal to the widthof the critical signal conductive paths, or the width may be determinedby the relative criticality of the signals. The more critical the signalborne by the conductive path is to the signal borne by the conductivevia, the wider the stripe and vice-versa.

The foregoing and other objectives, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of an illustrative embodiment when read in conjunction withthe accompanying drawings, wherein like reference numerals indicate likecomponents, and:

FIG. 1A is a cross-sectional view of a substrate in accordance with anembodiment of the present invention.

FIG. 1B is top view of the substrate of FIG. 1A.

FIGS. 2A-2G are cross sectional views illustrating steps in themanufacture of a substrate in accordance with an embodiment of thepresent invention.

FIG. 2H is a cross sectional view of an integrated circuit package inaccordance with an embodiment of the present invention.

FIG. 3 is a graph depicting a reflectometer display depicting aperformance improvement provided by the substrate of the presentinvention.

FIG. 4 is a graph depicting a reflectometer display depicting the effectof conductive strips of the substrate of the present invention onsignal-bearing via performance.

FIG. 5 is a pictorial diagram depicting a workstation computer system bywhich design methods and computer program products are executed inaccordance with an embodiment of the present invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

The present invention concerns integrated circuit package substrates andmethods of designing and making the substrates provide for routing ofconductive signal paths over signal-bearing vias, while solvingimpedance matching and isolation problems associated with prior artsubstrates. As in the above-incorporated U.S. patent application“MULTI-LAYER CIRCUIT SUBSTRATE AND METHOD HAVING IMPROVED TRANSMISSIONLINE INTEGRITY AND INCREASED ROUTING DENSITY”, metal reference planesare used to provide transmission line characteristics for signal pathsand voids are included in the metal reference planes over signal-bearingvias to prevent insertion capacitance mismatches. However, in thepresent invention, voids over which conductive signal paths are routedinclude conductive “stripes” extending across the vias in line with theconductive signal path, effectively splitting the voids into two voids(or more if multiple signal paths are routed over the via, as a stripcould be provided for each signal path). The stripes also reducecross-coupling between the signal path and the via, as the fieldsassociated with the signal path are more contained due to the presenceof the stripe.

The stripe width is generally equal to that of the conductive signalpath, but may be altered to reduce fringing effects on impedance or maybe tailored to the relative criticality of the individual signals on thevia and the conductive signal path. Since the ideal condition for properimpedance matching along the conductive signal path is the absence ofany void at all, and the ideal condition for eliminating insertioncapacitance at the ends of the void is the absence of the metalreference plane (or at least a complete void), the present inventioninvolves a trade-off between preserving the signal on the conductivesignal path and preserving the signal on the signal-bearing via.Therefore, since the width of the stripe is proportional to theperformance of one signal and inversely proportional to the othersignal, the relative criticality of the signals can be used to determinethe appropriate width.

The stripe is also generally centered in the void (or, in other terms,between the two partial voids formed by splitting the void), but that isnot a limitation of the present invention. Signal routing considerationsmay require an offset from a central diameter of the void, or thedirection of the signal path and stripe may change as the signal crossesthe void. In general, the shape and position of the stripe may reflectany shape and position of the conductive signal path in order tomaintain a level of impedance matching that is improved over theperformance obtainable in the absence of the stripe.

Referring now to FIG. 1A, an integrated circuit package substrate inaccordance with an embodiment of the present invention is shown. Thesubstrate includes a core 10 including through-via conductors providedby resin-filled plated-through hole (RFPs) 12A-12C. Metal layers areformed by plating, deposition or laminating on both sides of core 10containing jog stubs 14A-14C and areas of reference voltage plane layer11, with an insulating layer 15 laminated above stubs 14A-14C andreference voltage plane layer 11. A transmission line reference planemetal layer 17 is laminated, or otherwise deposited, above insulatinglayer 15 and a second insulating layer 19 is laminated, or otherwisedeposited, above transmission line reference plane metal layer 17. Asignal layer including signal path conductors 18 is laminated orotherwise deposited above insulating layer 19. For each criticalsignal-bearing RFP 12A, large-diameter voids 13 in transmission linereference plane metal layers 17 are provided above and below ends ofsignal-bearing RFPs 12A, which reduces the shunt capacitance fromsignal-bearing RFP 12A to transmission line reference plane metal layers17. In the present invention, a stripe 20 extends across the void 13above signal-bearing RFP 12A, which provides some shunt (insertion)capacitance, as mentioned above. However, stripe 20 reduces couplingbetween a signal path conductor 18A that is routed above signal-bearingRFP 12A, and further provides a much-improved impedance profile tosignal path conductor 18A.

While the void portions surrounding stripe 20 are devoid of metal, inpractice, the void portions will generally be filled with dielectric,lamination adhesive or other non-conductive material.

Signal-bearing RFPs 12A are connected to signal path conductors 18 bystubs 14A and small-diameter vias 16A. Without large diameter voids 13,the shunt capacitance from the ends of signal-bearing RFP 12A totransmission line reference plane metal layers 17 will cause signaldegradation greater than that caused by the presence of stripe 20.Voltage plane RFPs 12B and 12C (and optionally RFPs bearing non-criticalsignals) have no corresponding large-diameter voids in transmission linereference plane metal layer 17, which increases their distributedcapacitance by the shunt capacitance from RFPs 12B, 12C to transmissionline reference plane metal layer 17, which is generally desirable.Therefore, the stripes of the present invention are generally used oversignal-bearing vias, and generally only when a conductive signal path isrouted over a signal-bearing via.

Reference plane RFP 12B, which corresponds to the voltage plane to whichtransmission line reference plane metal layer 17 is connected, has astub 14B connecting to transmission line reference plane metal layer 17through a small via 16B. Blind vias connected to transmission linereference plane metal layer 17 can further be used in connections tosignal path layers added above the layer containing signal conductors18, to provide electrical connection to the particular voltage planeconnected to transmission line reference plane metal layer 17, ifneeded. Therefore, no void (and therefore, no stripe) is needed intransmission line reference plane metal layer 17 above reference planeRFP 12B. Other voltage plane RFPs 12C will generally require formationof vias 16C extending to other layers above transmission line referenceplane metal layer 17 from stubs 14C. Small-diameter voids 13A provideconnection to other voltage plane RFPs 12C and extend only above theends of stubs 14C, for signal routing channels above transmission linereference plane metal layer 17 above the top ends (and beneath thebottom ends for layers applied beneath core 10, not specifically shown)of other voltage plane RFPs 12C. The voltage plane used to provide areference to transmission line reference plane metal layer 17 may be apower supply voltage supplying the input/output drivers (the I/O signalreference and/or return voltage) or ground.

Referring now to FIG. 1B, a top view of the integrated circuit packagesubstrate of FIG. 1A is shown. Voids 13 are defined by transmission linereference plane metal layer 17, with additional metal removed abovesignal path stubs 14A and small diameter voids 13A for vias 16A thatconnect signal path stubs 14A to other signal layers. The resultingintegrated circuit package substrate has improved isolation betweensignal path conductors 18 routed over the continuous portions oftransmission line reference plane metal layer 17, while eliminating theshunt capacitance from signal-bearing RFPs 12A to metal layer 17, whenno stripe is present and reducing the shunt capacitance when a stripe ispresent. Large diameter via 12A is illustrated with a conductive signalpath 18A routed above and a stripe 20 included in void 13B. Stripe 20has been widened slightly for illustrative purposes, and another strip20A is shown without a corresponding conductive path or via for clarity.

Thus, in the present invention increased routing channels are providedin the regions extending over the top ends (or bottom ends) of all ofRFPs 12A, 12B and 12C. Thus, the substrate of the present inventionprovides improved signal performance in signal paths, providing forhigher processor or other VLSI circuit operating frequencies, whileproviding increased routing flexibility by providing more routingchannels that can have adequate signal performance no matter whethersignal paths are routed above core RFPs that carry power distributionand/or non-critical signals as described in the above-incorporated U.S.patent application, or routed above signal bearing RFPs such as RFP 12A.

Referring now to FIGS. 2A-2G, a method of making an integrated circuitsubstrate and integrated circuit in accordance with an embodiment of theinvention is shown. As shown in FIG. 2A, starting from a core dielectriclayer 40 having via holes 41 formed therein, holes 41 are filled withresin/metal to form PTHs 42. Stubs 44 and reference plane areas 43 areformed on both surfaces of core 40, as shown in FIG. 2B. An insulatinglayer 45 is then applied to one or both sides of the core dielectriclayer 40, over stubs 44 as shown in FIG. 2C. Next, insulating layer 45is opened to generate small-diameter via holes, forming insulating layer55. Then, metal is added in the small-diameter via holes to form smallvias 56 to connect to voltage plane RFPs as shown in FIG. 2D. Next, atransmission line reference plane metal layer 58 with voids 57 andstripes 58A-58C is applied as shown in FIG. 2E. Stripes 58A and 58Cillustrate stripe for conductive paths extending perpendicular to thepage of the Figure, and stripe 58B illustrates a stripe for a conductivepath extending along the plane of the Figure. Voids 57, including thevoid portions around stripes 58A-58C will generally be filled withdielectric insulating material or lamination adhesive as describedabove. Both the insulating layer 55 and transmission line referenceplane metal layer 58 may be applied as laminates, or the insulatinglayer may be deposited and/or transmission line reference plane metallayer 58 may be plated atop insulating layer 55. Voids 57 and stripes58A-58C may be pre-formed in transmission line reference plane metallayer 58 or etched. Next, as shown in FIG. 2F, another insulating layer60 is applied in a manner similar to that for insulating layer 55, andsmall voids 62 are formed or pre-formed in insulating layer 60 forconnection to signal RFPs. Finally, blind vias 64 and a signal layer 66are formed as shown in FIG. 2G that provide electrical connection tosignal RFPs. Conductive signal path 66A corresponding to stripe 58A,conductive signal path 66B corresponding to stripe 58B and conductivesignal path 66C corresponding to stripe 58C are also shown, which formpart of signal layer 66. Blind vias 64 and signal layer 66 may be formedat the same time, for example, by plating, or blind vias 64 may beformed first by filling or plating and then signal layer 66 laminated orplated to connect to blind vias 64.

Referring now to FIG. 2H, an integrated circuit in accordance with anembodiment of the present invention is shown. The substrate of FIG. 2Gis further modified by adding further signal layers, and optionallyvoltage plane layers on one or both sides of the core dielectric layer40. As illustrated another insulating layer 55A and signal layer 76A areadded, but in practice, numerous other layers may be added. Asemiconductor die 70 is attached to lands or other structures accessiblefrom the top layer of the substrate shown in FIG. 2G and terminals orlands (not shown) may similarly be added to the bottom side of thesubstrate after other circuit layers are added. Alternatively, lands canbe formed directly on the bottom side of core dielectric layer 40 orterminals may be attached to the bottom side of RFPs 42.

Referring now to FIG. 3 and FIG. 4, performance benefits and trade-offsof the present invention are shown. FIG. 3 illustrates a reflectiontrace 80A representing performance of a 28 μm wide conductive signalpath routed over a via that is 150 μm in diameter and including a 28 μmwide stripe under the conductive signal path. An improvement of 3 dB at10 GHz is shown (in reduction of reflection) over reflection trace 80B,which is for the same configuration without the stripe. However, asmentioned above, including the stripe changes the performance of thesignal-bearing via. FIG. 4 illustrates a reflection trace 82A for theconfiguration including the stripe, which is approximately 3 dB higherin reflection at 10 GHz for the configuration without the stripe, whichis illustrated by reflection trace 82B. However, as is noted from theFigures, the effect of the void/via on performance of the signal pathrouted over the via is greater than the effect of the stripe on theperformance of the signal path that includes the via. Therefore,absolute reflection level and improvement should be taken into accountin any design, as well as optionally the relative criticality of thesignals.

Referring now to FIG. 5, a workstation computer system 100 is shown inwhich the methods of the present invention are carried out in accordancewith an embodiment of the present invention, according to programinstructions that may be embodied in a computer program product inaccordance with a present invention, for example program instructionsstored on a CD-ROM disc CD. Workstation computer system includes aprocessor 102 for executing the program instructions coupled to a memory104 for storing the program instructions, data and results used indesigning integrated circuit substrates in accordance with embodimentsof the present invention. Workstation computer system 100 also includesperipheral devices such as CD-ROM drive 105 for reading discs such as CDin order to load the program instructions into workstation computer 100.Input devices, such as a keyboard 107A and a mouse 107B are coupled toworkstation computer system 100 for receiving user input. A graphicaldisplay 106 for displaying results such as the layout of metal layer 17of FIGS. 1A-1B, substrate layer designs as illustrated in FIGS. 2A-2Gand test data or simulations such as that of FIGS. 3-4. The depictedworkstation computer 100 is only exemplary and illustrates one type ofcomputer system and arrangement suitable for carrying out the designmethods of the present invention. The design methods generally identifythe locations of signal bearing vias and generate a mask design for atransmission line reference plane metal layer that includes voids aroundthe profile of the signal-bearing vias so that capacitive couplingbetween the ends of the signal-bearing vias and the transmission linereference plane metal layer is substantially reduced. The locations ofconductive signal paths routed over the signal-bearing vias is alsoidentified and stripes inserted to reduce coupling and match impedanceof the conductive signal path. The design methods may also consider therelative criticality of signals on the signal-bearing voids andconductive signal path and adjust the width of the stripes to optimizetrade-offs in performance.

While the invention has been particularly shown and described withreference to the preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in form,and details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A computer program product comprising acomputer-readable device storing program instructions, wherein theprogram instructions are program instructions for designing layers of asubstrate for mounting and interconnecting a semiconductor die, andwherein the program instructions comprise: program instructions forfirst identifying locations of signal-bearing vias from among a patternof large-diameter conductive vias extending from a top side to a bottomside of a core comprising a dielectric layer; program instructions forsecond identifying signal-bearing conductive path profiles for criticalsignals routed above or below ends of the signal-bearing vias; andprogram instructions for generating a first mask design for atransmission line reference plane metal layer including voids around theprofile of the signal-bearing vias so that capacitive coupling betweenthe ends of the signal-bearing vias and the transmission line referenceplane metal layer is substantially reduced, and wherein for thesignal-bearing conductive path profiles identified by the secondidentifying, the program instructions for generating include aconductive stripe extending through the corresponding void.
 2. Thecomputer program product of claim 1, further comprising programinstructions for further determining widths of the conductive stripes inconformity with a relative criticality of a first signal associated witha given signal-bearing conductive path profile identified by the secondidentifying to a criticality of second signal associated with acorresponding one of the signal-bearing vias, wherein a width of theconductive stripe corresponding to the given signal-bearing conductivepath profile path is increased for increased relative criticality of thefirst signal.